1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus such as an ultrasound diagnostic apparatus for medical treatment or an ultrasonic flaw detector for nondestructive testing, wherein an ultrasonic wave is propagated through a living body or a subject to obtain display image data items for ultrasonic diagnosis, such as an M-mode image (motion image), a B-mode image (tomographic image), a bloodstream velocity image, a CFM image (color flow mapping image), etc.
2. Description of the Related Art
This type of ultrasonic imaging apparatus has some problems to be solved.
One of the problems is that the receiving delay quantization precision is low. In other words, a receiving echo signal produced by each ultrasonic transducer element includes components of various frequencies and has a wide dynamic range. Thus, in the receiving delay processing for beam deflection or beam focusing in a receiving system, each receiving echo signal supplied from each element needs to be subjected to a receiving delay in steps of about 10 nsec.
It is possible to perform this receiving delay processing in a digital manner, by connecting an A/D converter to each of simultaneous receiving elements, directly A/D converting a receiving echo obtained from each element, using a shift-register or the like as a digital delay circuit formed of a semiconductor memory element, and controlling the data readout time in steps of 10 nsec.
However, the quantization precision of delay is determined by the conversion rate of the A/D converter. Thus, in order to achieve the quantization precision of delay of 10 nsec, it is necessary to provide the same number of A/D converters functioning by conversion clocks of 1/10 nsec=100 MHz as the number of simultaneous receiving elements. The provision of such A/D converters involves high costs, and power consumption is high. Thus, it is very difficult to manufacture an ultrasonic imaging apparatus by using such A/D converters. On the other hand, if inexpensive A/D converters having a low conversion rate (e.g., 50 nsec) are used, the resolution of the apparatus would be degraded, and artifacts increase. The use of these inexpensive A/D converters is also unpractical.
Under these circumstances, an analog delay circuit (analog delay line) comprising a coil inductance L and a capacitance C is generally used for receiving delay processing, and a precision of delay time of about 10 nsec is maintained.
The analog receiving delay processing using the above analog delay circuit has the following drawbacks: undesirable frequency characteristic, artifact due to crosstalk, disturbance of signal waveform due to reflection, variation in delay time, etc.
Owing to these drawbacks, imaging data for B-mode images or the like is deteriorated.
Another desirable goal is to obtain a fine ultrasonic image by increasing the number of scanning lines, and to improve the real time characteristic by increasing the number of frames without changing the number of scanning lines.
As a technique solving this problem, a multidirection simultaneous receiving system has been proposed, which comprises a plurality of receiving delay circuits, and a plurality of summing circuits and receivers corresponding to the receiving delay circuits. Ultrasonic beams having a plurality of receiving directivities are received by a single probe.
In this system, if the analog delay circuits are used for the above-stated reason, it becomes impossible to simultaneously set a plurality of receiving directivities. Under these circumstances, it is necessary to provide the same number of receiving delay systems as the number of simultaneous receiving directivities, resulting in increase in circuit scale of the apparatus.